Patent Document 1, for example, discloses a centrifugal fan such as a turbofan having a plurality of blades, which are arranged between a main plate and a side plate (a shroud). FIGS. 31 to 34 each illustrate a turbofan employed in the indoor unit of a ceiling embedded air conditioner.
With reference to FIGS. 31 to 34, an indoor unit 1 of a ceiling embedded air conditioner has a cassette type body casing 2, which is embedded in a ceiling 3. An air inlet/outlet panel 4 is arranged at a lower surface of the body casing 2. The air inlet/outlet panel 4 is substantially flush with the ceiling 3.
A rectangular air inlet grill 5 is arranged at the center of the air inlet/outlet panel 4. A bellmouth 6 of a turbofan 11 is arranged at the backside of the air inlet grill 5 in the body casing 2. A plurality of air outlet ports 9 each having a predetermined width are formed in the air inlet/outlet panel 4 and outside the air inlet grill 5.
An air passage 10, which extends from the air inlet grill 5 to the air outlet ports 9 through the bellmouth 6, is formed in the body casing 2 along the entire circumference of the body casing 2. The turbofan 11 is suspended from a ceiling panel 2a of the body casing 2 through a fan motor 13. The turbofan 11 is arranged at the backside (the upper side as viewed in FIG. 31) of the bellmouth 6 in the air passage 10. The turbofan 11 has a side plate 15, which is arranged at the air inlet side. The side plate 15 of the turbofan 11 is arranged to face the bellmouth 6. An air heat exchanger 12 is arranged in the air passage 10 so as to surround the turbofan 11.
The turbofan 11 has a circular main plate (hub) 14, the side plate (a shroud) 15 having a tubular shape, and a plurality of blades (movable blades) 16, which are arranged between the main plate 14 and the side plate 15. The main plate 14 is fixed to a rotary drive shaft 13a of the fan motor 13. The blades 16 are arranged at predetermined blade angles and spaced apart at predetermined intervals in a circumferential direction. The side plate 15 has two opening ends having different outer diameters. One of the opening ends of the side plate 15 forms an air inlet port that guides air in centrifugal directions in an impeller. An air outlet port portion 6c of the bellmouth 6 is loosely arranged in an air inlet end portion 15a of the side plate 15. The bellmouth 6 is arranged rotatably with respect to the side plate 15 with a predetermined clearance maintained between the bellmouth 6 and the side plate 15.
After air has been drawn through the air inlet grill 5, the bellmouth 6 causes the air to smoothly flow in the centrifugal directions with respect to the air inlet end portion 15a of the side plate 15. Specifically, as illustrated in FIG. 31, the bellmouth 6 extends horizontally inward from an attachment portion 6a, which is attached to the air inlet/outlet panel 4, and extends vertically in such a manner that the diameter of the opening of the bellmouth 6 becomes smaller from upstream to downstream. The bellmouth 6 has an air inlet port portion 6b and the air outlet port portion 6c. The air inlet port portion 6b and the air outlet port portion 6c each form an airflow guide surface having a predetermined radius of curvature. The bellmouth 6 has an arcuate cross section along the airflow guide surface. Since the bellmouth 6 is shaped in this manner, the bellmouth 6 smoothly guides the air, which has drawn into the turbofan impeller, in the centrifugal directions with respect to the side plate 15 of the turbofan impeller. This minimizes the fan noise caused by air. As has been described, in the centrifugal fan such as the turbofan, the airflow guide surfaces of the bellmouth 6 and the side plate 15 are formed to have ideal shapes so as to reduce air turbulence occurring in an outer circumferential portion or an inlet portion of the impeller, thus reducing the noise caused by the air.
In a configuration disclosed in Patent Document 2, in order to improve the air blowing performance, only an end of a leading edge 16a of each blade 16 close to the side plate 15 is inclined in the rotating direction of the blade 16. This prevents separation of the airflow produced on a negative pressure surface at an inlet of the blade 16.
However, as illustrated in FIG. 35, the side plate 15 disclosed in Patent Document 1, which is shown in FIGS. 31 to 34, has an arcuate cross section having a predetermined radius of curvature, which extends from the air inlet end portion 15a to an air outlet end portion 15b. The arcuate surface extending from the leading edge 16a of each blade 16 to a trailing edge 16b of the blade 16 is slightly twisted. The blade 16 extends linearly from the main plate 14 in the vertical direction. Accordingly, an extremely small sharp corner area having a V-shaped cross section is formed between the inner arcuate surface (the airflow guide surface) of the side plate 15 and the blade 16. The corner area forms a dead water region, which is a factor decreasing the speed of the airflow. This deteriorates the original performance of each blade 16. The problem cannot be solved even by inclining only the leading edge 16a of the blade 16 in the rotating direction as described in the configuration disclosed in Patent Document 2.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2001-115991    Patent Document 2: Japanese Laid-Open Patent Publication No. 10-196591